JCM800 split channel mod Ottovalvole

[otto]

This is the ultimate mod for the jcm800 split channel. I've reached it through pc simulation, circuit comparison and tests. The mod improves the clean channel but above all the crunch one, taking off diodes without losing distorsion. Above all it makes the sound fit both for rithm (as it was) and solo playing, giving the typical feeling of a full tube circuit.
If you try the mod please let me know what you think about.
You find the scheme in images at link: www.myspace.com/ottovalvole
Keep attention: touching tube circuits can be lethal also with power switched off, if you aren't skilled in the subject and you don't know how to discharge the capacitors don't touch the amp and let the work be done by someone of the job.

[Doug H]

cool. but I don't have a myspace account.

[drewl]

That's in a funny language.

[Doug H]

I translated it, Otto just talks about it in general, the pics are what you want, I don't have a myspace account so I can't see them to verify.

btw, it's Italian. I tried German first thinking Otto was a German name.

[otto]

Sorry about that.
I post here the schem

[Doug H]

I like how you setup v1a into the lead signal, I read your other post a while back googling my amp. If I ever remove the diodes I'll probably give that a try too. Wouldn't there be some kind of voltage drop or something? I guess it doesn't matter, jsut let out the grid resistors a bit, and hey, more gain is more gain

[otto]

I like how you setup v1a into the lead signal

Thanks. I'm proud of this simple idea, it's there ...
Re-designing the circuit around this idea solves all the troubles with this circuit and its discussed diodes

Wouldn't there be some kind of voltage drop or something?

no problems with mine

jsut let out the grid resistors a bit, and hey, more gain is more gain

I think the gain obtained is the max one for this number of stages, grid resistors used are designed to balance gain between stages

If you will try the mod don't forgive to write a feedback ...

[otto]

I've tranlated in English the myspace page.
www.myspace.com/ottovalvole

[meballar]

Please tell me the values of the new components

Hi Otto,
Thank you for posting your recommendations to improve the sound of the early 4210. Can you please tell me the values of the new components, it's hard to read some of the values and I don't want to make a mistake! I thought about buying a 4010 amp, but maybe it's worth the time to first try the modifications. After the modifications, does the amp sound like a 4010 or is it a unique sound. It would be nice if you could make a youtube video of your amp. Thanks bro.
Mike

[otto]

Hi Mike. You should ask me which values you cannot read ...
At the moment I have no occasion to make a sample.
The circuit is different respect a single channel like the 4010 so it hasn't exactly the same sound. The crunch channel should be something like a JCM800 single channel 'pumped up'.
What I can say is that respect the standard 4210 you can hear the preamp tubes sound whithout losing crunch and together with this you have a nice clean channel (not a Fender one due to the amp section)

[Alexmc1988]

Is it possible for me to get some detailed instructions for this mod? I have never modded an amp before, only pedals. where would be a good place to start?

[otto]

Is it possible for me to get some detailed instructions for this mod? I have never modded an amp before, only pedals. where would be a good place to start?

Hi Alex.
I repeat here that touching tube circuits can be lethal also with power switched off, if you aren't skilled in the subject and you don't know how to discharge the capacitors don't touch the amp or read something about this subject before, to learn how to do.
With this attention in mind I can say to you that the power section is not modded, only the preamp, with the difference respect to pedals that it is a tube circuit, but the mod consists in adding or replacing resistors and capacitors (or blocks of them) so there is not much difference in the work.
There is alto to interrupt the circuit at one point, this is a mechanical job, you have to cut the line on the board.
But maybe you could start with a simpler mod ...
Try to ask some suggestion about your doubt in a more general thread in this forum ...
Good luck

[Alexmc1988]

Does the schematic that isn't hand drawn have mods done to it to because it looks a little different from the original schematic I got off a different web site for the 4210

[Alexmc1988]

OK so I am fully confident that I can do this mod. I have gone threw the schematics and highlighted all of the changes that I can find. I must admit this is a bigger mod that I thought it was going to be but I am still confident. I have all of next week off and then I am moving back to school so I will have time to work on it. I know how to discharge the amp by placing alligator clips on the first pin of a pre amp tube then on to the chassis or some other ground. I am just having trouble following the schematics. For one Are the letters, such as "A" were the lines just stop will I make a connection to a solder pad or trace there and the then trace goes to where the other "A" is? I am also not sure where you have cut the board. It looks like the reverb is cut out of the signal switching. Will I still be able to turn the reverb on and off with the foot switch? Also I to am having trouble reading a lot of the values are you sure you can't just make a parts list? Also is there additional information on the hand drawn schematic? It looks incomplete to me and I don?t know what the "TS" with a circle around it means. Also what wattage of resistors will I need and do you suggest metal film caps and resistors like with effects pedals?
Thanks Alex.

[otto]

Does the schematic that isn't hand drawn have mods done to it to because it looks a little different from the original schematic I got off a different web site for the 4210

in internet you find one schematic for pre-1987 version and one for post-1987
(see www.drtube.com)
my mod is for the first one